Screw-threaded pipe joint

ABSTRACT

A screw-threaded joint for a pipe has a male portion ( 10 ) and a female portion ( 12 ), having complementary screw threads ( 11, 13 ). The male thread ( 11 ) extends to a position adjacent a complementary stop shoulder ( 18 ) on the female portion ( 12 ) having a recess of conical cross-section having a rounded apex. The recess is adapted to receive a head portion having a conical cross-section with a rounded apex, the radius of curvature of which is greater than that of the recess&#39;s apex providing a seal with improved stress dispersal properties upon application of torque. The wall ( 33 ) of the cone&#39;s in-use outwardly facing surface on the male portion ( 10 ) can subtend an angle of from 28-36° to the longitudinal axis of the pipe. The corresponding cone wall on the female portion can subtend an angle of from 28-38° with the same axis. The male portion ( 10 ) can comprise a second stress dispersing surface ( 41 ), concave in shape.

FIELD OF THE INVENTION

The present invention relates to a connection for joining pipestogether, in particular to pipes for use underground in, for example,the gas and oil industry.

BACKGROUND OF THE INVENTION

When transporting a fluid, such as oil or natural gas between twolocations, the most cost effective methods, where possible utilisepipelines. Such pipelines are normally constructed from individual pipesjoined together. The regions where the pipes join, constitutediscontinuities in both the inner and outer surface of the pipeline andas such are sources of weakness within the finished pipeline.

It is important that any such weakness is minimised as much as possible,as any loss of fluid from the pipe is both wasteful of resources andpotentially disastrous for the environment. Furthermore, the fluid lostcan also increase the risk to personnel from fire or explosion.Replacement of inefficient or leaking joints is not always easy.Particularly in the case of the oil and gas industry, pipelines aresituated quite often in relatively inaccessible locations andreplacement is difficult and expensive.

A number of solutions have been employed to overcome weakness. The mostcommon solution is a screw-threaded joint, the threads of the jointacting together to prevent separation of the individual pipes eitheraxially or radially.

The joints formed must firstly be so constructed that any fluids withinthe pipe cannot escape. They must also be capable of withstanding largedifferentials in pressure across the radial width of the joint, whichpressure differential usually has a positive component in an outwardlyradial direction.

Screw-threaded joints require the engagement of two threaded regionssituated one on each of the pipes to be connected together. Engagementis normally by application of a torque. The two threaded regions arenormally manufactured so that their threads are complementary to eachother.

GB 1 587 836 (VALLOUREC) discloses a screw connection for pipes in whicha stop-shoulder on one pipe co-operates with a complementarystop-shoulder on the other pipe to form a seal.

There a number of problems with such a seal. Firstly, the metal fromwhich the stop-shoulders are formed, flows under the high torque appliedwhen screwing the pipes together. Therefore if care is not takenslippage or buckling of the joint can occur. Secondly, the joints formedcan comprise a step discontinuity in going from one section of pipe tothe other.

It is an object of the present invention to provide a screw-threadconnection which addresses the above problems

SUMMARY OF THE INVENTION

According to the present invention there is a provided a screw threadedjoint for pipes comprising a first pipe length having at one end a malescrew threaded portion and a second pipe length having at one end afemale portion having a complementary screw thread, the portions beingadapted to inter-engage alone the greater part of the axial length ofthe threaded portions, the screw threads thereof being inclined in thesame direction and at an acute angle to the longitudinal axis of thepipe length, at least the male thread extending to a position adjacent acomplementary stop shoulder on the other portion, characterised in thatthe or each complementary stop shoulder comprises a recess in the formof a cone having a rounded apex, the recess being adapted to sealinglyreceive a head in the form of a cone having a rounded apex with a radiusmarginally greater than the radius of the recess thereby providing asealing surface with improved stress dispersal properties uponapplication of a torque to the joint.

The wall of the cone on the in-use outer surface of the malescrew-threaded portion preferably subtends an angle of from 28 to 36° tothe longitudinal axis of the pipe. Particularly preferably the wall ofthe cone subtends an angle of from 30 to 33° to the longitudinal axis ofthe pipe, and most preferably an angle of 30°.

The corresponding cone wall on the female threaded portion preferablysubtends an ankle of from 28-38° with the longitudinal axis of the malethreaded portion. The angle subtended by the cone wall of the femalethreaded portion is preferably greater than that subtended by the conewall of the male threaded portion.

The male screw threaded portion preferably comprises a stress dispersingsurface connecting the stop-shoulder to the first thread of the pin. Thestress dispersing surface preferably describes a concave curve.

The radius of curvature of the concave curve of the stress dispersingsurface is preferably from 0.075 to 0.085 cm, and is most preferably0.081 cm.

The male screw threaded portion preferably comprises an end-shoulderwhich subtends an angle of from 14-24° with the plane perpendicular tothe longitudinal axis of the pipe. The end-shoulder preferably subtendsan angle of from 18-20°. It particularly preferably subtends an angle of20°.

The end-shoulder of the female threaded portion preferably subtends anangle of 12-24°. With the plane perpendicular to the longitudinal axisof the male threaded portion. The angle subtended is conveniently from14-18°. The angle subtended by the end-shoulder of the female threadedportion is preferably less than that subtended by the end-shoulder ofthe male threaded portion.

When the connection is axially compressed, the male screw threadedportion has a tendency to bulge inwards due to inwardly directed radialforces generated by the compression. The deformation thus caused isoften known as barrel deformation.

Advantageously the in-use inner surface of the stop-shoulder ispreferably concave to reduce the effects of barrel deformation. Theradius of the concave surface is preferably from 4.9 to 5.2 cm, and mostpreferably 5.1 cm.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described more particularly with reference tothe accompanying drawings which show, by way of example only twoembodiments of a screw-thread connector for a pipe. In the drawings:

FIG. 1 is an axial sectional view of the screw-thread connection betweena pin and a box;

FIG. 2 is a larger scale axial sectional view of the stop-shoulder ofthe pin of FIG. 1;

FIG. 3 is an axial sectional view of a second embodiment of thestop-shoulder of a pin; and

FIG. 4 is an axial sectional view of a pin and box joint.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring initially to FIG. 1, a pin 10 comprising a screw-thread 11, isthreadably engaged with a box 12 having a corresponding screw-thread 13.The engagement of the pin 10 with the box 12 forms a joint. The pin 10and the box 12 are tapered towards their respective ends 14, 15 suchthat the total width of the pipe wall in the region 16 remainssubstantially constant. A stop-shoulder 17 on the pin 10, engages acorresponding stop-shoulder 18 on the box 12 to form a seal.

In use the two screw-threads 11, 13 are engaged by application of atorque to the two pipes, until the seal is formed by contact between thesurfaces 19, 20, 21 of stop-shoulder 17, with the surfaces 22, 23, 24respectively of the stop-shoulder 18.

The stop-shoulder of the pin 10 is illustrated in more detail in FIG. 2.The pin 10 has at one end a substantially frusto-conical stop-shoulder31. The stop-shoulder 31 has a number of surfaces 32, 33, 34 whichengage the corresponding stop shoulder of the box. Between the surface33 and the end-shoulder 34 the stop shoulder is curved, having a radiusof curvature of 0.036 cm. The surface 33 subtends an angle of 30° withthe longitudinal axis of the pin 10. Furthermore, the end-shoulder 34subtends an angle of 20° with the plane perpendicular to thelongitudinal axis of the pin 10. The stop shoulder comprises a headportion, the head portion having a conical cross-section with a curvedapex, the conical section being defined by side walls 33 and 34.

The surface 37 which connects the wall 35 of the thread 36 to thesurface 32 is curved. The surface 37 has a radius of curvature of 0.081cm.

FIG. 3 shows a further embodiment of the pin described above in FIG. 2.In this embodiment, the pin 40 comprises a further curved surface 41,having a radius of curvature of 5.08 cm. The curvature is such that whena torque is applied to the pin 40 whilst connecting it to a box, thestress induced by the torque causes an inward radial force to bend thepin 40 so that the surface 41 becomes flush with the box. In this way,flow of a fluid through the pipe is not disrupted by projections ordiscontinuities in the pipe.

Such disruption, perhaps resulting in vortices within the fluid flowresults in more energy being required to transport the fluid alone thepipe. Furthermore, discontinuities are usually more vulnerable thanother parts of a surface to chemical or physical attack from thetransported fluid, with the result that they are more labile towardscorrosion by the fluid.

FIG. 4 illustrates a further aspect of a pin and box joint according tothe present invention. Surface 51 of the pin 50 has a radius ofcurvature of 0.036 cm. This radius of curvature is greater than that ofthe corresponding curve surface 52 on the box 53, which has a radius ofcurvature of 0.030 cm. The resulting gap 54, caused by the difference incurvature can be utilised during the coupling process. As surfaces 51and 52 do not come into contact with each other until the pin and boxare correctly engaged, stress in these regions is kept to a minimum foras long as possible. Once the pin 50 and box 53 are correctly engaged,as further torque is applied, the material from which the pipe is formedcan begin to experience plastic flow due to the high strain to which itis subjected. The gap 54 can be utilised by allowing material undergoingsuch plastic forces to flow into it. Apart from this improving the seal,the flowing of the material into an empty space does not set up anyadditional strains in the box or pin. This would normally be the caseshould the shoulder of the pin not be able to flow fairly freely. Afurther advantage of the difference in curvature is that it can allowmore easily, inward movement of the sealing region of the joint, whichagain reduces unnecessary stress.

The features described above can either individually or in conjunctionwith one another, reduce areas of high stress normally encounteredwithin a stop-shoulder when the pipe on which it is located is connectedto a second pipe. The features allow the energy transferred to thestop-shoulder on application of the torque to be dissipated more widelyalong the stop-shoulder region, and into the thread region.

Although not wishing to be limited by any particular theory it isbelieved that incorporation of the features discussed hereinredistributes stresses brought about by the application of torque to thejoint. Approximately 80-95% of the stress is concentrated in the sealingarea of the coupling. The energy being stored in this region reducesunwanted plastic yielding of the material from which the pipes areformed.

Moreover, as a result of consistent control of the interferencethroughout the thread, particularly on the root of the pin, the crest ofthe coupling and the load flanks and with the improved seal geometry,the connection can receive a higher torque, which in turn improves thecompression performance of the connection.

Although as illustrated in FIG. 4, the walls 55, 56 of the pin areparallel to and define the same angle as the walls 57, 58 of the boxthis need not be the case, and in many embodiments of the invention adifferent angle is advantageous. For example in a preferred embodiment(not illustrated), the surface of the female threaded portioncorresponding to the surface 33 in FIG. 2 subtends an angle of 32° andthat surface corresponding to the shoulder 34, subtends an angle of 18°.

In general the surface 33 can subtend an angle of from 28-36° with thelongitudinal axis of the longitudinal axis of the pipe, whilst thecorresponding surface on the female threaded portion can subtend anangle of from 28-38°.

Similarly, the shoulder 34 can subtend an angle of from 14-24° with theplane perpendicular to the longitudinal axis of the pipe, whilst thecorresponding shoulder on the female threaded portion can subtend anangle of from 12-24°.

The difference in the angles reduces the bending stress in the seal areaon application of a torque, and allows the energy of the torque to beused in a more efficient way.

The stop-shoulders as disclosed herein can be employed when formingpipes from many industry standard materials.

The pipes are typically be formed from steel. The grade of steel usedfor the pipe can be selected from those normally used in the pipelineindustry. For example the following may be cited as suitable for use inthe invention,—using standard nomenclature—K55, J55, N80, L80, C90, C95,P110 and Q125. The pipes may also be subject to anti-galling surfacetreatment prior to their being used.

The overall width of the pipe is typically from 3.8 cm to 12.7 cm intotal diameter. It is more typically from 6.0 cm to 11.5 cm.

It will of course be understood that the invention is not limited to thespecific details described herein, which are given by way of example,and various modifications and alterations are possible within the scopeof the invention

What is claimed is:
 1. A screw-threaded joint for pipes comprising afirst pipe length having a male screw threaded portion at one end and asecond pipe length having a female portion at one end, the end having acomplementary screw thread, the threads being adapted to inter-engagealong a greater part of the axial length thereof the threads beinginclined in the same direction and at an acute angle to the centrallongitudinal axis of the joint, the male thread extending at least to astop shoulder having a head portion which is positioned adjacent acomplementary stop shoulder on the female portion, characterised in thatthe complementary stop shoulder comprises a cone receiver having afrusto-conical cross-section and a rounded receiver apex, the conereceiver adapted to sealingly receive the head portion which has afrusto-conical cross-section and rounded apex of radius greater than theradius of the apex of the cone receiver, thereby providing a sealingsurface upon application of a torque to the joint and in that the in-useinner surface of the stop shoulder of the male screw-threaded portion isconcave when viewed from the inside of the pipe.
 2. A screw-threadedjoint according to claim 1, characterised in that the in-use outwardlyfacing cone wall of the head portion subtends an angle of from 28 to 36°with respect to the longitudinal axis of the joint.
 3. A screw-threadedjoint according to claim 2, characterised in that the cone wall subtendsan angle of from 30 to 33° with respect to the longitudinal axis of thejoint.
 4. A screw-threaded joint according to claim 1, characterized inthat the in-use inwardly facing cone wall on the cone receiver subtendsan angle of 28 to 38° with respect to the longitudinal axis of thejoint.
 5. A screw-threaded going according to claim 1, characterized inthat the angle subtended by the in-use inwardly facing cone wall of thecone receiver is greater than that subtended by the in-use outwardlyfacing cone wall of the head position.
 6. A screw-threaded jointaccording to claim 1, characterized in that the male screw-threadedportion comprises a second stress dispersing surface connecting the stopshoulder to the thread of the pipe.
 7. A screw-threaded joint accordingto claim 6, characterised in that in cross-section the second stressdispersing surface describes a concave curve interacting longitudinallywith the stop shoulder.
 8. A screw-threaded joint according to claim 7characterised in that the radius of curvature of the concave curve ofthe second stress dispersing surface is from 0.075 to 0.085 cm.
 9. Ascrew-threaded join according to claim 8, characterised in that theradius of curvature of the concave surface of the second stressdispersing surface is from 0.081 cm.
 10. A screw-threaded jointaccording to claim 1, characterized in that the male screw threadedportion comprises an end-shoulder which subtends an angle of from 14 to24° with respect to the plane perpendicular to the longitudinal axis ofthe pipe.
 11. A screw-threaded joint according to claim 10,characterised in that the end-shoulder subtends an angle of from 18 to20° with respect to the plane perpendicular to the longitudinal axis ofthe pipe.
 12. A screw-threaded joint according to claim 1, characterizedin that the female threaded portion comprises and end-shoulder whichsubtends an angle of 12-24° with respect to the plane perpendicular tothe longitudinal axis of the male threaded portion.
 13. A screw-threadedjoint according to claim 12, characterised in that the end-shoulder ofthe female threaded portion subtends an from 14 to 18° with a planeperpendicular to the longitudinal axis of the male threaded portion. 14.A screw-threaded joint according to claim 12, characterized in that theangle subtended by the end-shoulder of the female threaded portion isless than that subtended by the end-shoulder of the male threadedportion.
 15. A screw-threaded joint according to claim 1, characterizedthe in-use inner surface of the stop shoulder of the male threadedportion is concave when viewed from the inside of the pipe.
 16. Ascrew-threaded joint according to claim 15, characterised in that theradius of the concave surface is from 4.9 to 5.2 cm.
 17. Ascrew-threaded joint according to claim 16, characterised in the radiusof the concave surface is 5.1 cm.